Ceria slurry for polishing semiconductor thin layer

ABSTRACT

A non-agglomerating ceria aqueous slurry, which has good dispersion stability and reduced generation of scratches with excellent polishing productivity, comprises ceria particles having a mean volume particle size ranging from 0.1 to 0.2 μm and is characterized in that when it is centrifuged under an applied average centrifugal force (g) of 1,970 g 0  for 2 minutes, g 0  being acceleration of gravity, the amount of the solid content reduction is 20% by weight or less.

FIELD OF THE INVENTION

The present invention relates to a ceria slurry for polishing semiconductor thin layer, having good dispersion stability and excellent polishing performance without generating scratches.

BACKGROUND OF THE INVENTION

Aqueous ceria (cerium oxide) slurries have been widely used for chemical mechanical polishing (CMP) of a semiconductor or insulating thin layer, but conventional ceria slurries have disadvantage that ceria particles settle down during a long-term storage and agglomerate when re-dispersed, thereby generating large particles. Accordingly, many attempts have been made to improve the dispersion stability of the ceria slurry with the aid of dispersants and dispersing apparatus.

Recently, it has also necessary to develop a ceria slurry which generates no scratches as the connecting circuit patterns formed on the semiconductor layer and the gap between the semiconductor chips became minimized. Japanese Laid-open Patent Publication No. 2003-171653 suggested a ceria slurry comprising secondary ceria particles having a particle size of greater than 3 μm in an amount of less than 1%, but this slurry still generates scratches and it is not suitable for polishing a wafer for a micropattern having a narrow line width of below 0.16 μm.

Therefore, it is required to completely remove abrasive particles responsible for the generation of scratches from a ceria polishing slurry, and recently, it has been shown that even a trace amount of particles having a particle size of greater than 700 nm, particularly greater than 1 μm, actually generate scratches on the surface of a wafer.

In this connection, Japanese Laid-open Patent Publication No. 2003-188122 suggested a ceria slurry comprising abrasive particles having a volume average particle size of 1 to 95 nm wherein the particles of a size greater than 0.56 μm are present in a concentration of less than 100,000/ml. However, this slurry has the problems of a low polishing rate and poor planarization characteristics due to excessively fine abrasive particles.

SUMMARY OF THE INVENTION

Accordingly, it is a primary object of the present invention to provide a ceria slurry comprising a markedly reduced level of large-sized particles, and thus, having good dispersion stability, excellent polishing performance characteristics that can be advantageously used in the CMP (chemical mechanical polishing) of semiconductor and electroluminescent devices.

In accordance with one aspect of the present invention, there is provided a ceria aqueous slurry for polishing a semiconductor thin layer, comprising ceria particles having a mean volume particle size ranging from 0.1 to 0.2 μm, the slurry being characterized in that when it is centrifuged under an applied average centrifugal force (g) of 1,970 g₀ for 2 minutes, g₀ being acceleration of gravity, the amount of the solid content reduction is 20% by weight or less.

DETAILED DESCRIPTION OF THE INVENTION

The inventive polishing ceria slurry is characterized in that when it is subjected to centrifugation under a applied average centrifugal force (g) of 1,970 g₀ (g₀=acceleration of gravity; 9.8 m/sec²) for 2 minutes, the amount of the solid content reduction is 20 wt % or less, and thus, it is substantially free of ceria particles having a mean particle size of greater than 0.7 μm which generate scratches on a substrate for forming a micro-pattern having a line width of below 0.16 μm.

In accordance with the present invention, the ceria slurry has a mean volume particle size ranging from 0.1 to 0.2 μm as measured by a laser scattering particle size distribution analyzer, and a D₁₀₀ ranging from 0.5 to 0.7 μm, D₁₀₀ denoting the particle size at the point the accumulated value of particle size distribution becomes 100 (i.e., the particle size of the largest particle present in the slurry).

The inventive ceria aqueous slurry may be prepared by dispersing a ceria powder in water and then centrifuging the resulting aqueous slurry until the solid content reduction change value of the slurry as defined above becomes below 20 wt %.

Specifically, at first, a ceria powder is dispersed in water to a concentration of about 0.5 to 20 wt % in accordance with any one of the conventional procedures, e.g., ultra-sonic, wet mill and high-pressure impact dispersion methods. The starting ceria powder may be commercially available or prepared in a conventional procedure, e.g., by calcining cerium carbonate, hydroxide, nitrate, chloride or acetate at 650 to 900° C. to obtain ceria and powdering the ceria thus obtained with a wet or dry mill, to obtain a ceria powder having an average particle diameter of 10 to 100 nm. The calcining and milling processes may be carried out in a reverse order.

A dispersant may be preferably used in the preparation of the ceria slurry. The resulting ceria slurry may have a pH of 4 to 9. The dispersant may be selected based on the surface potential value of ceria in water, and an anionic compound is preferably used. Representative examples of the dispersant are poly(acrylic acid), poly(vinylsulfuric acid), poly(methacrylic acid), polyacrylamide and polyarylamine. The dispersant may preferably have a weight average molecular weight of 1,000 to 50,000. If the molecular weight is less than 1,000, the ceria slurry becomes unstable, while if the molecular weight is greater than 50,000, the viscosity of the ceria slurry becomes too high for good storage stability. The dispersant may be conventionally used in an amount ranging from 0.1 to 10 wt % based on the ceria used. When the amount of the dispersant is lower than 0.1 wt %, the stability of the slurry becomes poor, whereas when the amount of the dispersant is higher than 10 wt %, non-adhered dispersants remain in the slurry to make the storage stability of the slurry poor.

In accordance with the present invention, the ceria slurry thus homogeneously dispersed is then subjected to a centrifuging process. In the present invention, the ceria slurry is centrifuged under a several-thousand rpm, preferably 1,000 to 5,000 rpm, in order to forcibly remove relatively large particles having a particle size of greater than 700 nm. The centrifuging may be conducted using a cylindrical centrifuge through which the slurry is passed in a manner that the slurry is charged into a bottom portion and then vented from the top portion of the rotating centrifuge to force relatively large-size particles present in the slurry to stick to the inside wall of the centrifuge.

The ceria slurry is centrifuged until it has a solid content reduction change of 20 wt % or less, preferably 10 wt % or less, as tested under the specified centrifuging condition, as mentioned previously. The solid content change value defined in the present invention makes it possible to evaluate the relative contents of large particles present in given two slurries having the same mean volume particle size.

The slurry thus centrifuged may further be filtered with a filter in a conventional manner, if necessary.

The inventive slurry satisfying the above solid content change requirement has excellent long-term storage stability, thus preventing agglomeration of particles and lowering the number of scratches generated during the polishing process by 90% or more, and further, it has an average particle size sufficient to polish a substrate with good polishing productivity. Therefore, the inventive slurry can be beneficially used in a CMP process of a wafer for forming a micro-pattern having a narrow line width of below 0.16 μm, as well as in a CMP of an interlayer insulating film or shallow trench isolation (STI).

The present invention is further described and illustrated in the following Examples, which are, however, not intended to limit the scope of the present invention.

Preparation 1: Preparation of Ceria Powder

Ceria hydroxide was oxidized at 750° C. to obtain a ceria powder, which was milled with a ball mil to obtain a ceria powder having an average particle diameter of about 40 nm as measured with an XRD (X-ray diffraction) analyzer.

Preparation 2: Preparation of Layer for Polishing

A silicon dioxide layer was formed on 8 inch silicon wafer to a thickness of 10,000 Å according to PE-TEOS (Plasma enhanced-tetraethyl orthosilicate) method to obtain a layer for polishing.

Preparation 3: Preparation of Ceria Slurry

800 g of the ceria powder obtained in Preparation 1 was dispersed in 9,260 g of de-ionized water and stirred using a propeller stirrer for 30 minutes, to obtain a ceria aqueous suspension. 20 g of a dispersant containing a poly(acrylic acid) having a weight average molecular weight of 3,000 at a concentration of 40 wt % was added to the ceria suspension obtained above with stirring. During the reaction, the pH of the solution was maintained at 4-8 using diluted aqueous ammonia or nitric acid containing no alkali ion. The resulting solution was made into a slurry under a pressure of 200 Mpa using a high- pressure wettable disperser to obtain an 8 wt % ceria slurry.

Preparation of Polishing Slurries

EXAMPLE 1

The ceria slurry obtained in Preparation 3 was centrifuged through a cylindrical centrifuge rotating at a speed of 1,500 rpm, and adding de-ionized water thereto, to obtain a 5 wt % ceria slurry according to the present invention.

EXAMPLE 2

The procedure of Example 1 was repeated except that the rotating speed was changed to 2,000 rpm, to obtain a 5 wt % ceria polishing slurry.

EXAMPLE 3

The procedure of Example 1 above was repeated except that the obtained ceria slurry was further filtered through a 1 μm filter after centrifuging, to obtain a 5 wt % ceria polishing slurry.

COMPARATIVE EXAMPLE 1

The ceria slurry obtained in Preparation 3 was simply diluted with de-ionized water, without centrifuging, to obtain a 5 wt % ceria polishing slurry.

COMPARATIVE EXAMPLE 2

The ceria slurry obtained in Preparation 3 was filtered through a 3 μm filter, without centrifuging, and diluted with de-ionized water, to obtain a 5 wt % ceria polishing slurry

COMPARATIVE EXAMPLE 3

Ceria hydroxide was oxidized at 650° C. to obtain a ceria powder, which was milled with a ball mil to obtain a ceria powder having an average particle diameter of about 25 nm as measured with an XRD (X-ray diffraction) analyzer. This ceria powder was made into a slurry as in Preparation 3 and centrifuged and diluted to obtain a 5 wt % ceria polishing slurry

Evaluation of Solid Content Change Depending on Centrifuging Condition

The ceria slurry of Example 1 was tested to determine the change in the solid content (the extent of solid content decrease, wt %) when centrifuged under various conditions, and the test results are represented in Table 1. TABLE 1 Rotating Times speed 1 min 2 min 3 min 5 min 1,000 rpm 0%   0%   0% 0.7% 2,000 rpm 0% 1.1%  2.0% 3.6% 4,000 rpm 1.1%   8.2% 13.3%  25% Evaluation on Properties of Polishing Slurries

The slurries of Examples 1 to 3 and Comparative Examples 1 to 3 were examined with a particle size distribution analyzer LA910 (a product of Horiba in Japan), and the results are shown in Table 2.

Further, in order to examine the content of particles which may agglomerate during a long-term storage, the polishing slurries of Examples 1 to 3 and Comparative Examples 1 to 3 were subjected to a centrifuging test with a tubular centrifuge (Hanil Science Industrial MF 80, Korea), to remove the precipitated particles, and the changes in the solid content were determined.

Specifically, 40 ml of each of the 5 wt % ceria slurries was weighed and filled into a test tube having a height of 11. 5 cm and a volume of 50 ml, and centrifuged at 4,000 rpm (at which the distance from the central axis of the centrifuge to the bottom of the test tube became 14 cm, and the distance from the central axis of the centrifuge to the top of the test tube became 2.5 cm, and thus, the centrifugal force applied to the slurry became 1,970 g₀) for 2 minutes to obtain a supernatant, which was weighed to determine the solid content change weight percentages of the slurries. The results are represented in Table 2. The change in the solid content, i.e., the amount of removed particles by the centrifuging in wt %, increases when the amount of relatively large particles present in the slurry increases.

Further, using each of the ceria slurries obtained in Examples 1 to 3 and Comparative Examples 1 to 3, the layer for polishing obtained in preparation 2 was polished with Mirra equipment (a product of AMAT Inc., USA), which is a CMP polisher for 8 inch wafer, and IC1000/suba IV stacked pad (a product of Rodel Inc., USA) under a pressure of 3.5 psi for 90 seconds. The polishing slurry was fed at a rate of 150 ml/min, and the spin rate of the upward plate was 104 rpm, while the spin rate of the downward plate was 110 rpm.

After polishing, the number of scratches having a size of more than 0.16 μm generated on the layer was determined using AIT-XP (a product of KLA Tenco, Inc., USA), and the polished amount was measured using Therma-wave Optiprobe 300 series (a product of Therma-wave, USA). The results are also listed in Table 2. TABLE 2 Number of Solid content scratches Polished Mean particle reduction (number/ Amount size (nm) wt % wafer) (Å/min) Ex. 1 163 8.2 30 4530 2 122 7.7 19 4270 3 158 6.2 7 4480 Com. 1 172 28.5 245 4660 Ex. 2 165 21.2 186 4510 3 85 7.6 28 980

As can be seen from Table 2, the ceria slurries in accordance with the present invention have excellent dispersion stability and generates a remarkably reduced level of scratches at a sufficiently high polishing rate, as compared with the prior slurries.

While some of the preferred embodiments of the subject invention have been described and illustrated, various changes and modifications can be made therein without departing from the spirit of the present invention defined in the appended claims. 

1. A ceria aqueous slurry for polishing a semiconductor thin layer, comprising ceria particles having a mean volume particle size ranging from 0.1 to 0.2 μm, the slurry being characterized in that when it is centrifuged under an applied average centrifugal force (g) of 1,970 g₀ for 2 minutes, g₀ being acceleration of gravity, the amount of the solid content reduction is 20% by weight or less.
 2. The ceria aqueous slurry of claim 1, which is obtained by dispersing a ceria powder in water to a concentration of 0.5 to 20 wt % and centrifuging the resulting slurry at a rotation speed of 1,000 to 5,000 rpm.
 3. The ceria aqueous slurry of claim 2, wherein centrifuging is carried out by passing the slurry through a cylindrical centrifuge.
 4. A method for polishing the surface of a thin film layer of a semiconductor or insulating material using the ceria aqueous slurry of claim
 1. 5. The method of claim 4, wherein the semiconductor thin layer is employed for a micropattern having a line width of below 0.16 μm. 